The present invention relates generally to the art of photovoltaic devices, such as solar cells for converting light into electrical energy, and, more particularly, to a flexible photovoltaic device.
Photovoltaic devices generally consist of an active photovoltaic material capable of generating an electrical potential upon being exposed to light, contacts, which are effective to draw off any electric current which results from irradiation of the active photovoltaic material, and a suitable substrate. In most instances, such photovoltaic devices are relatively rigid, either because the active photovoltaic material itself is rigid or because the substrate or other component of the device is inflexible. Glass, which is relatively inflexible, has found frequent use as substrate in photovoltaic devices because of its strength, durability, tolerance to high processing temperatures and desirable optical properties. Attempts to provide flexible photovoltaic devices have been documented. A flexible rolled up solar array, for example, has been described in "The Flight of the FRUSA" by George Wolff and Alois Wittmann, Conf. Rec., Ninth IEEE Photovoltaic Specialists Conf., May 2-4, 1972, page 240-253, reprinted in Solar Cells, edited by Charles E. Backus, The Institute of Electrical and Electronics Engineers, Inc., New York, 1976, pp. 342-355.
Many different photoconductive materials, such as silicon, germanium, gallium arsenide and copper indium diselenide, for example, are used in solar cells and similar photovoltaic devices. Photoconductors comprising silicon have obtained particularly wide usage because of their economy. Originally, single crystal silicon photoconductors were widely used. However, recently, thin film Si:H alloys (TFS) have been used in thin film solar cells, that is, solar cells fabricated from microcrystalline, amorphous compound semiconductors or semiconductor material other than single crystalline semiconductor material, deposited in situ upon a substrate. Such thin film solar cells have come to be preferred because of their lower cost, ease of fabrication, and durability.
For example, active photovoltaic materials incorporating a P-layer, an I-layer and an N-layer, known as P-I-N photovoltaic materials, have been utilized. The I-layer, the intrinsic layer, is generally formed from a thin layer of microcrystalline or amorphous silicon alloyed with hydrogen. The P-layer is formed from a silicon-hydrogen alloy doped with boron or similar dopant. The N-layer is formed from a silicon-hydrogen alloy doped with phosphorous or similar dopant.
Various types of conductive layers or contacts have been used in photovoltaic devices. Contacts on the P-layer and the N-layer, for example, permit electric current to be drawn off into an external circuit for use therein. Such contacts usually have taken the form of thin films. In addition, transparent conductive layers, such as tin oxide, indium tin oxide, or other transparent conductive oxides have found application in solar cells and other photovoltaic devices. The use of zinc oxide films as transparent conductive layers, for example, is disclosed in U.S. Pat. No. 4,623,601 issued Nov. 18, 1986 to Steven C. Lewis et al, assigned to the same assignee as the present invention. Such conductive oxides are relatively transparent in reasonable thicknesses and have been used primarily as the front contact in solar cells, that is, the contact adjacent to the transparent substrate, which may face or be distant from the incoming light rays. They have also been used as back contacts in solar cells where transparency is a major consideration, for example, in tandem cells or bifacial cells.
The utilization of thin film photoconductors and thin conductive layers has made available solar cells which are thinner and lighter in weight than previously available solar cells. However, presently available thin film solar cells have only limited flexibility, due primarily to the use of glass as a substrate. Rather than providing a truly flexible solar cell, the prior art has approached the problem by providing a rolled up array of inflexible solar cells upon a flexible backing, as in the FRUSA system.
It is accordingly an object of the present invention to provide a flexible photovoltaic device.
It is another object of this invention to provide such flexible photovoltaic device which can be manufactured in a continuous process.
Another object of this invention is to provide a flexible photovoltaic device having good optical properties.
Other objectives and advantages of the present invention will become apparent from the following detailed description.